Trypsin is a serine protease that hydrolyzes proteins in two sequential steps. The first acylation occurs when the nucleophilic serine 195 attacks the substrate at the scissle bond (bond later to be broken). The tetrahedral intermediate is formed by the His 57 forming a hydrogen bond with the substrate polypeptide. His 57 itself is stabilized by a hydrogen bond from Asp 103 (along with Ser 195 forming the catalytic triad). The various conformation changes often leaves His 57 represented as two ring complexes. This representation is due to its nearly equally abundant conformations it is found in. Next, the C-terminal fragment is released, leaving only a covalently bound acyl-enzyme. A second deacylation reaction occurs after a water molecule attacks the acyl-enzyme. This series of events leads to the formation of a second terminal intermediate. Then, the N-terminal fragment, which is often stabilized by the oxyanion hole, is released. The histidine residue acts as a base in the first step by accepting a proton to activate serine as a nucleophile. The histidine later acts as an acid by donating the proton to the nitrogen of the peptide leaving group. [1]
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Structural highlights
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